A major obstacle to placing a controversial Montgomery County landfill in the Laytonsville area was removed yesterday when a judge ruled a county charter amendment banning such dumps in residential areas is "invalid and void."

The ruling by Montgomery Circuit Court Judge Philip Fairbanks came in litigation over the 550-acre, upper county site officials want as a repository for the 1,300 tons of trash that county residents generate each day. The location of future trash disposal sites in the county has been bitterly debated for a decade.

State Del. Robin Ficker (R-Montgomery) and five other citizens had filed suit challenging the legality of the Laytonsville site and asking for $19 million in damages. They argued that the dump would be a nuisance and a health hazard. The county filed a countersuit.

Fairbanks declared invalid the so-called Ficker amendment to the county charter, which states that "no expenditure of county funds shall be made or authorized for the operation of a landfill system or refuse disposal on lands zoned for residential use."

Ficker pushed the amendment and collected enough signatures to have a referendum on it placed on the 1978 election ballot. The amendment was approved by a 2-to-1 margin.

The judge ruled that the amendment "constitutes legislation enacted in violation of the Constitution of Maryland, the state Express Powers Act and the County Charter." He said a law reserves for Montgomery and Prince George's counties the power to designate projects such as landfills.

"As of right now, there are absolutely no lawsuits pending against the Laytonsville landfill," said Nathan Greenbaum, the assistant county attorney who argued the case for the county. "But it's probably just a lull in the storm."

Ficker agreed, saying the decision would be appealed and criticizing the judge's decision as "a weasel-worded political statement. The judge is telling 70 percent of some of the most intelligent people in the country to drop dead.

"All of the arguments the judge made were contrived after the election," Ficker said. "That amendment won every precinct and got more votes than the public officials who were running at the time. It's like trying to tell people who elected County Executive Charles W. Gilchrist that he can't serve."